Case fatality rate
In epidemiology, a case fatality rate (CFR) — sometimes called case fatality risk or case-fatality ratio — is the proportion of deaths from a certain disease compared to the total number of people diagnosed with the disease for a particular period. A CFR is conventionally expressed as a percentage and represents a measure of disease severity. CFRs are most often used for diseases with discrete, limited-time courses, such as outbreaks of acute infections. A CFR can only be considered final when all the cases have been resolved (either died or recovered). The preliminary CFR, for example, during an outbreak with a high daily increase and long resolution time would be substantially lower than the final CFR.
The mortality rate — often confused with the CFR — is a measure of the relative number of deaths (either in general, or due to a specific cause) within the entire population per unit of time. A CFR, in contrast, is the number of dead among the number of diagnosed cases only.
Sometimes the term case fatality ratio is used interchangeably with case fatality rate, but they are not the same. Case fatality ratio is the comparison between two different case fatality rates, expressed as ratio. It is used to compare different diseases or to assess the impact of interventions.
From a mathematical point of view, CFRs, which take values between 0 and 1 (or 0% and 100%, i.e., nothing and unity), are actually a measure of risk (case fatality risk)— that is, they are a proportion of incidence, although they don't reflect a disease's incidence. They are neither rates, incidence rates, nor ratios (none of which are limited to the range 0-1). They do not take into account time from disease onset to death.
Infection fatality rateEdit
Like the case fatality rate, the term infection fatality rate (IFR) also applies to infectious disease outbreaks, but represents the proportion of deaths among all infected individuals, including all asymptomatic and undiagnosed subjects. It is closely related to the CFR, but attempts to additionally account for inapparent infections among healthy people. The IFR differs from the CFR in that it aims to estimate the fatality rate in both sick and healthy infected: the detected disease (cases) and those with an undetected disease (asymptomatic and not tested group). (Individuals who are infected, but show no symptoms, are said to have "unapparent", "silent" or "subclinical" infections and may inadvertently infect others.) By definition, the IFR cannot exceed the CFR, because the former adds asymptomatic cases to its denominator.
If 100 people in a community are diagnosed with the same disease, and 9 of them subsequently die from the effects of the disease, the CFR would be 9%. If some of the cases have not yet resolved (neither died nor fully recovered) at the time of analysis, a later analysis might take into account additional deaths and arrive at a higher estimate of the CFR, if the unresolved cases were included as recovered in the earlier analysis. Alternatively, it might later be established that a higher number of people were infected with the pathogen, resulting in an IFR lower than the CFR.
The following examples show some CFRs for diseases in the real world:
- The CFR for the Spanish (1918) flu was <2.5%, but about 0.1% for the Asian (1956-58) and Hong Kong (1968-69) flus, and <0.1% for other influenza pandemics.
- The current best estimate of the CFR for COVID-19 according to the Johns Hopkins University is 2.8% in the US.
- Legionnaires' disease has a CFR of ≈15%.
- Yellow fever has a CFR of about 3–7.5%.
- Bubonic plague has the best prognosis of the three main variants of plague, but if left untreated, has a CFR >60%.
- Ebola virus is among the deadliest viruses, with a CFR as high as 90%.
- Naegleriasis (also known as primary amoebic meningoencephalitis), caused by the unicellular Naegleria fowleri, nearly always results in death, with a CFR >99%.
- Rabies virus is almost invariably fatal if left untreated, and so has a CFR approaching 100%; however, patients generally respond well to prompt post-exposure prophylaxis.
- Prion diseases are progressive and always fatal, regardless of treatment.
References and notesEdit
- Rebecca A. Harrington, Case fatality rate at the Encyclopædia Britannica
- For example, a diabetes mortality rate of 5 per 1,000 or 500 per 100,000 characterizes the observation of 50 deaths due to diabetes in a population of 10,000 in a given year. (See Harrington, Op. cit..)
- "Coronavirus: novel coronavirus (COVID-19) infection" (PDF). Elsevier. 2020-03-25. Archived from the original (PDF) on 2020-03-27. Retrieved 2020-03-27.
- Bosman, Arnold (2014-05-28). "Attack rates and case fatality". Field Epidemiology Manual Wiki. ECDC. Archived from the original on 2020-03-25. Retrieved 2020-03-25.
- Entry “Case fatality rate” in Last, John M. (2001), A Dictionary of Epidemiology, 4th edition; Oxford University Press, p. 24.
- Hennekens, Charles H. and Julie E. Buring (1987), Epidemiology in Medicine, Little, Brown and Company, p. 63.
- "Infection fatality rate". DocCheck Medical Services GmbH. Retrieved 25 March 2020.
- "Global Covid-19 Case Fatality Rates". Centre for Evidence-Based Medicine. Retrieved 25 March 2020.
- Taubenberger, Jeffery K.; David M. Morens (January 2006). "1918 influenza: the mother of all pandemics". Emerging Infectious Diseases. Coordinating Center for Infectious Diseases, Centers for Disease Control and Prevention. 12 (1): 15–22. doi:10.3201/eid1201.050979. PMC 3291398. PMID 16494711. Archived from the original on 2009-10-01. Retrieved 2009-04-17.
- Li, F C K; B C K Choi; T Sly; A W P Pak (June 2008). "Finding the real case-fatality rate of H5N1 avian influenza". Journal of Epidemiology and Community Health. 62 (6): 555–559. doi:10.1136/jech.2007.064030. ISSN 0143-005X. PMID 18477756. S2CID 34200426. Retrieved 2009-04-29.
- "" Johns Hopkins University, October 5th 2020.